Quantum entanglement and the 2022 Nobel Prize

The 2022 Nobel prize in Physics on quantum entanglement has been announced and the findings are incredible! In short, what researchers Alain Aspect, John Clauser, and Anton Zeilinger have discovered is that entangled particles behave like a single unit even when they are separated far far away. In this article we will discuss this finding and attempt to help you come to a better understanding of this discovery and its potential.

Visualization of quantum entanglement between two particles. Image courtesy of NobelPrize.org

Classical Physics Breaks Down at Smaller Scales

To begin understanding the 2022 Nobel prize earning discovery we must discuss physics at very small scales. The sizes we must consider are in the atomic and subatomic range. As we know the tenets of classical physics break down as we reduce the dimensions to quantum scale – roughly characterized by the order of the magnitude of De Broglie wavelength. Turns out, tiny objects start behaving much like waves rather than solid objects as we approach this scale.

To internalize this concept, you can visualize the “breaking down” of our intuitive understanding using small particles such as dust. Consider when you observe dust in a room illuminated by a ray of sunlight. Have you ever noticed that what you are seeing as the “ray” is actually sunlight bouncing off of small dust particles in the air.

An image of a sun ray as it illuminates levitating dust. Image courtesy of StockAdobe.com

With this knowledge you may ask yourself “how dust is being levitated in the air?”. Our intuition normalizes gravity being the dominant force over drag. However, for such small objects because their mass to surface ratio is so in favor of the surface the result is drag being enough to counteract gravity at a near zero velocity. Hopefully this analogy makes you feel more comfortable with the idea that the world indeed changes at smaller scale.

Quantum Entanglement and the EPR Paradox

Things get much weirder as we approach atomic and subatomic scales. One such weirdness is the EPR Paradox, which is a thought experiment summarized in a rather famous 1935 paper by Einstein, Podolsky and Rosen. The paradox involved a hypothetical pair of quantum particles which were a priori prepared in a entangled quantum state. For example consider a pair of particles resulting from a split of a bigger particle (system) with zero momentum. By the law of momentum conservation if one of the sibling particles possessed a momentum of 1, the other would necessarily posses -1. They argued that if we could predict the value of one particle then with certainty we would also know the value of the second particle.

This viewpoint assumes that a-priori each of those particles have definite values of momentum. However quantum mechanics argues that the nature of particles is described by a wave function and each particle have both values with certain probability. Moreover, the act of measuring of one particle collapses the wavefunction of that particle to one determinate state. By the law of momentum conservation, this  implies that we would also collapse the wavefunction of the other particle, even though the second particle could have been light-years away from us. 

Einstein, Podolsky and Rosen argued that this cannot be true as it would violate the principles of special relativity according to which no information can travel with a speed of light. Einstein referred to this impossibility as “spooky action at a distance”. He further argued that instead the states of the particles are determined a-priori and there must be some “hidden” variables which pre-determined their final quantum states before they were separated. This viewpoint essentially suggested that the quantum mechanics was incomplete at best or wrong at worst.

Their view point was of a world which is deterministic and which is firmly grounded in the principle of locality. While EPR paradox did not coin the term “quantum entanglement”, it set out a quest for better understanding of the phenomenon that would unravel only decades later. In it’s quest to understanding this paradox better, John Bell came up with his famous theorem called “Bell’s Theorem”.

What is Bell’s Inequality?

John Bell a physicist from Northern Ireland was intrigued by the EPR paradox and expanded on it by recognizing that the deterministic, locality principle based view of universe was incompatible with the probabilistic view that quantum mechanics offered. In particular, EPR view implied that there must exist some “hidden variables” – hypothetical properties of quantum particles, which are not yet detectable by us but which still affect the outcome of experiments. Bell basically stipulated that if these hidden variables are local to each particle then the EPR view will fundamentally be incompatible with the quantum mechanics.

More specifically, he concluded that if indeed some “hidden variables” determine the outcome of measurements of these entangled particles at a distance then there should also be a mathematical constraint on how the outcomes on the two measurements are correlated. For a deeper understanding of Bell’s inequality watch this video from IBM Quantum:

Hopefully this video clarified that Bell’s inequality stems from hidden variable theories about entanglement. These theories assume hidden variables or information is shared at the moment of entanglement. Hidden variable theories imply that there is a mathematical limit to how correlated measurements of entangled particles can be. This limit is called Bell’s inequality. John Stewart Bell (the physicist after whom Bell’s theorem and inequality are named) also showed that quantum physics predicted correlations that violate this inequality.

Now back to the Nobel Prize

In short, Alain Aspect, John Clauser, and Anton Zeilinger performed experiments that proved the violation of this inequality in triumph of quantum mechanics. In doing so, they furthered our understanding of quantum entanglement. They did this by developing experiments that violate Bell’s inequality.

The experiments made by Alain Aspect, John Clauser, and Anton Zeilinger show that particles can interact after they’ve been apart. The notion of interaction while never touching violates the theory of hidden variables. Additionally, experimental measurements have shown correlation between results that exceed the mathematical limit posed by bells inequality.

These are the experiment concepts that were used to violate Bell’s inequality. Image courtesy of NobelPrize.org

Let’s briefly discuss why these experiments were so groundbreaking.

• John Clauser created base experiments that measured entangled photons multiple times. Using polarization as his measurement, he creates a clear violation of Bell’s inequality. This disrupts speculations and theories that use hidden variables.
• Alain Aspect developed a more advanced setup, using it in a way that closed an important loophole that existed in Clauser’s experiments. He was able to switch the measurement settings after an entangled pair had left its source, so the setting that existed when they were emitted could not affect the result.
• Anton Zeilinger further developed this kind of experiment and demonstrated a phenomenon called quantum teleportation. This phenomenon is where the changing of one entangled particle’s state will change its entangled pair’s state at a distance. This is “teleportation” because the state changes are instant and nothing physically moves from one place to another.

Implications to Quantum Computing

The groundbreaking discovery offers a much deeper understanding of entanglement and quantum mechanics in general, which could allow us to harness these effects in various technologies. All of this research and discovery is laying the foundation for new branches of science in quantum information. This is the basis for quantum computation, the transfer and storage of quantum information, and algorithms for quantum encryption.

At the moment, quantum information exists in the form of measurements. Humanity searches for a way to convert these measurements into information we can use analogously to bytes or signals. Hopefully we can harness quantum entanglement manipulate information similarly to how we have harnessed electricity to make machines.

A foundation in quantum information leads to the next concept: quantum computing – the concept of performing calculations and computer logic using quantum mechanics. Quantum entanglement may become a tool to create incredibly complex computers capable of equally complex computations. IBM already lists entanglement as one of the mechanisms used to make quantum computers work. Having increased understanding of entanglement is fundamental in creating these groundbreaking technologies.

This 2022 Nobel Prize highlights an important milestone in our understanding of quantum mechanics. We are in an age of humanity where quantum technology is on the horizon.